Micro-electrical-mechanical system (MEMS) devices come in a variety of types and are utilized across a broad range of applications. One type of MEMS device that may be used in applications such as radio frequency (RF) circuitry is a MEMS vibrating device (also known as a resonator). A MEMS resonator generally includes a vibrating body in which a piezoelectric layer is in contact with one or more conductive layers. Piezoelectric materials acquire a charge when compressed, twisted, or distorted. This property provides a transducer effect between electrical and mechanical oscillations or vibrations. In a MEMS resonator, an acoustic wave may be excited in a piezoelectric layer in the presence of an alternating electrical signal, or propagation of an elastic wave in a piezoelectric material may lead to generation of an electrical signal. Changes in the electrical characteristics of the piezoelectric layer may be utilized by circuitry connected to a MEMS resonator device to perform one or more functions.
Guided wave resonators include MEMS resonator devices in which an acoustic wave is confined in part of a structure, such as in the piezoelectric layer. Confinement may be provided on at least one surface, such as by reflection at a solid/air interface, or by way of an acoustic mirror (e.g., a stack of layers referred to as a Bragg mirror) capable of reflecting acoustic waves. Such confinement may significantly reduce or avoid dissipation of acoustic radiation in a substrate or other carrier structure.
Various types of MEMS resonator devices are known, including devices incorporating interdigital transducer (IDT) electrodes and periodically poled transducers (PPTs) for lateral excitation. Examples of such devices are disclosed in U.S. Pat. Nos. 7,586,239 and 7,898,158 assigned to RF Micro Devices, Inc. (Greensboro, N.C., USA), wherein the contents of the foregoing patents are hereby incorporated by reference herein. Devices of these types are structurally similar to film bulk acoustic resonator (FBAR) devices, in that they each embody a suspended piezoelectric membrane. Suspended piezoelectric membrane devices, and particularly IDT-type membrane devices, are subject to limitations of finger resistivity and power handling due to poor thermal conduction in the structures. Additionally, IDT-type and PPT-type membrane devices may require stringent encapsulation, such as hermetic packaging with a near-vacuum environment.
Plate wave (also known as lamb wave) resonator devices are also known, such as described in U.S. Patent Application Publication No. 2010-0327995 A1 to Reinhardt et al. (“Reinhardt”). Compared to surface acoustic wave (SAW) devices, plate wave resonators may be fabricated atop silicon or other substrates and may be more easily integrated into radio frequency circuits. Reinhardt discloses a multi-frequency plate wave type resonator device including a silicon substrate, a stack of deposited layers (e.g., SiOC, SiN, SiO2, and Mo) constituting a Bragg mirror, a deposited AlN piezoelectric layer, and a SiN passivation layer. At least one resonator includes a differentiation layer underlying a piezoelectric layer and arranged to modify the coupling coefficient of the resonator so as to have a determined useful bandwidth.
A representative MEMS guided wave device 10 of a conventional type known in the art is shown in FIG. 1. The device 10 includes a piezoelectric layer 12 arranged over a substrate 14, with top side electrodes in the form of an IDT 18 bounded laterally on two ends by a pair of reflector gratings 20. The IDT 18 includes electrodes with a first conducting section and a second conducting section that are inter-digitally dispersed on a top surface of the piezoelectric layer 12. The IDT 18 and the reflector gratings 20 include a number of fingers 24 that are connected to respective bus bars 22. For the reflector gratings 20, all fingers 24 connect to each bus bar 22. For the IDT 18, alternating fingers 24 connect to different bus bars 22, as depicted. (Actual reflector gratings 20 and IDT 18 may include larger numbers of fingers 24 than illustrated.) For the IDT 18, the fingers 24 are parallel to one another and aligned in an acoustic region that encompasses the area in which the IDT 18 and its corresponding reflector gratings 20 reside. At least one wave is generated when the IDT 18 is excited with electrical signals (e.g., supplied via bus bars 22). Acoustic waves essentially travel perpendicular to the length of the fingers 24 and essentially reside in the acoustic region encompassing the area including the IDT 18 and the reflector gratings 20. The operating frequency of the MEMS guided wave device 10 is a function of the pitch (P) representing the spacing between fingers 24 of the respective IDT 18. The wavelength λ of an acoustic wave transduced by the IDT 18 equals two times the pitch or separation distance between adjacent electrodes (fingers 24) of opposite polarity, and the wavelength λ also equals the separation distance between closest electrodes (e.g., fingers 24) of the same polarity.
In any of the above-described devices, access to exposed portions of an active region of a piezoelectric layer is limited, since an active region is typically obscured by presence of electrodes such as IDTs.
Additionally, it may be difficult to adjust one or more properties of a guided wave device, such as frequency, coupling coefficient, temperature compensation characteristics, velocity, phase, capacitance, or propagative wave mode, over portions or an entirety of a guided wave device. It may also be difficult to integrate one or more functional structures with a guided wave device without interfering with placement of electrodes such as IDTs.
Accordingly, there is a need for guided wave devices that can be efficiently manufactured, and that enable production of devices with enhanced utility.